Beta-Hydroxydecanoylthioester dehydrase ("dehydrase") is the enzyme from anaerobic and facultative microorganisms which functions at the branch point between biosynthesis of saturated and unsaturated fatty acids. It catalyzes dehydration of the common biosynthetic intermediate (3R)-3-hydroxydecanoyl acyl carrier protein to the E-2-decanoyl thioester and isomerization of the latter to the Z-3-decanoyl thioester. In an effort to shed further light on the mechanisms of these reactions we are studying their stereochemical aspects. The project requires synthesis of regio- and stereospecifically deuterated substrates, incubation with dehydrase, isolation of products, and analysis of label content and position. We are also studying, by similar methods, the stereochemistry of the isomerization catalyzed by an allene-acetylene isomerase recently isolated from hog liver.